Surface-induced symmetry breaking leads to unexpected vibrational activity of melem on Cu(111)

The adsorption of melem on Cu(111) at 80 K has been studied by Scanning Tunnelling Microscopy (STM), Reflection Absorption Infrared Spectroscopy (RAIRS) and Density Functional Theory (DFT) calculations. STM images and DFT calculations confirm a planar adsorption geometry and a triangular footprint at the surface, with the nitrogen atoms positioned above atop surface sites and the carbon atoms in 3-fold fcc sites. DFT calculations show that melem is adsorbed at a height of 3 Å above the surface, consistent with molecule-metal distances observed for physisorbed aromatic molecules. A small charge transfer of 0.27e^- and a small tilt of the NH₂ groups towards the surface are observed in the DFT calculations, endorsing the general physisorption model, albeit with a small perturbation. Simulated RAIRS spectra generated from the DFT model clearly indicate the reduction of symmetry from D_3h in the gas phase to C_3v in the adsorbed state, which causes specific gas phase modes to be switched on or off. The simulated RAIR spectrum is dominated by out-of-plane vibrations as expected for a planar physisorbed molecule. In contrast, the experimental RAIRS data show unexpectedly intense and broad, shifted bands arising from the in-plane NH₂ scissors + ring stretching vibrational modes, suggesting the presence of significant additional intermolecular and molecule-metal interactions in the adsorbed state that lead to bond softening and increased anharmonicity in the vibrational modes.